A pipeline transports oil and natural gas produced from an oil well and a gas well. Conventionally, a carbon steel has been used mainly for a steel pipe (line pipe) constituting a pipeline.
In recent years, however, as the well depth increases, portions known as a gathering line and a flow line of the line pipe are likely to be exposed to a corrosive environment having higher temperature and pressure than the conventional environment. Also, these portions must transport a produced fluid containing corrosive gases such as hydrogen sulfide gas and carbonic-acid gas. Therefore, the line pipe used for the gathering line and flow line is increasingly required to have excellent carbonic-acid gas corrosion resistance and sulfide stress-corrosion cracking resistance (hereinafter, sulfide stress-corrosion cracking is referred to as SSC).
In this situation, a martensitic stainless steel pipe for a line pipe has been developed as a steel pipe that meets the above-described requirement. The martensitic stainless steel pipe for a line pipe has been disclosed, for example, in JP3156170B.
The martensitic stainless steel pipe for a line pipe is provided with excellent carbonic-acid gas corrosion resistance and SSC resistance by forming a passivation film on the surface thereof by the addition of Mo and making the C content lower than 0.01%. Also, by containing a large amount of Ni as an austenite forming element substituting for C, the micro-structure can be kept martensitic even if the C content is low. Further, since the C content is low, work hardening is less liable to occur at the time of welding, and excellent weldability is demonstrated. Therefore, the martensitic stainless steel pipe for a line pipe is suitable to the use for the gathering line and flow line.
The pipeline includes not only a straight line pipe (so called a straight pipe) but also a line pipe having a curved portion, that is, a bend pipe according to the geographical features of the ground on which the pipeline is laid.
A general process for producing a bend pipe consisting of carbon steel, which has been used for the conventional pipeline, is described below. First, a straight pipe is bent at a high temperature into a bend pipe. Subsequently, the bend pipe is quenched and tempered. Since the mechanical properties such as strength and toughness of the bend pipe are deteriorated by the bending at a high temperature, the mechanical properties are improved by quench and temper.
As the well depth increases in recent years as described above, the martensitic stainless steel for a line pipe has begun to be used for the gathering line and flow line in place of carbon steel. Therefore, in place of the conventional bend pipe consisting of carbon steel, demand emerges for the bend pipe consisting of the martensitic stainless steel for a line pipe.
However, in the case where the bend pipe consisting of the martensitic stainless steel for a line pipe is produced in the same producing condition as that of the conventional bend pipe consisting of carbon steel, the SSC resistance of the produced bend pipe is sometimes low.